Progressive addition spectacle lenses are visual aids used in the management of presbyopia, the condition wherein the accommodative function of the eye is partially or fully lost. The Vision Council of America defines a progressive lens as a lens designed to provide correction for more than one viewing distance in which the power changes continuously rather than discretely. The power change of a progressive lens may be derived by modifying the surface curvature of a lens or the refractive index of the optical material comprising the lens, or both. A number of gradient index lens types have been proposed for use as progressive addition lenses. These lenses provide a change or gradient of refractive power over what may be termed a progressive intermediate vision or transition zone of the lens through a corresponding change in refractive index of the optical media comprising the lens, theoretically providing the advantage of reducing or avoiding the astigmatism associated with non-rotationally symmetric aspheric surface contours common to conventional progressive addition lenses. Due to problems associated with these designs, including issues and difficulties relating to manufacturing, there has been no commercialization of gradient index progressive power lenses. In order to provide adequate power for both distance and near vision functioning, a significant amount of refractive index change in the optical material is required. Ion exchange methods, proposed by some to achieve a refractive index change, may tend to offer both an undesirable gradient index profile and less than the needed power change for a progressive addition spectacle lens. Lenses produced by diffusion methods likewise have failed to provide adequate add power or realize commercial success.
U.S. Pat. No. 3,485,556 to Naujokas describes a multifocal plastic ophthalmic lens wherein there is provided a major lens portion of one index of refraction and a minor lens portion of a different index of refraction with a uniform index gradient therebetween. The plastic materials are produced by a process in which an interface is established between monomeric liquids and diffused over time in an isothermally controlled environment and then polymerized.
This lens at first appears to be capable of providing the stated distance and near vision properties. A ray tracing of the lens in accordance with the parameters set forth in the patent shows that only when a significantly high plus power configuration is utilized can an add power of even 1 diopter be achieved. Using the refractive indices of 1.5 and 1.6 identified in the patent a calculated plus power of 4.714 diopters is needed in the distance vision portion to achieve the only slightly greater power of 5.714 diopters in the near vision portion, thus the lens is useful only to those needing high plus power correction for distance vision. Furthermore, if a prescription incorporating cylinder power is produced on either the front or back surface, the cylinder power will vary and cause aberrations as a result of the changing refractive index.
U.S. Pat. No. 5,042,936 to Guilino et al. describes a progressive ophthalmic lens comprising a distance portion, the refractive power of which being designed for distance vision, a reading portion, the refractive power of which being designed for near vision, and an intermediate portion, in which the refractive power along the main line of vision at least partially increases continuously from the refractive power of the distance portion to the reading portion. A refractive index of the lens material varies along the main line of vision at least in the intermediate portion so as to at least partially contribute to the increase in refractive power and correction of aberrations.
According to the specification, each of two progressive ophthalmic lenses in front of the left or right eye is provided with a main point of vision (distance reference point) Bf for distance vision and a main point of vision (near reference point) Bn for near vision. Furthermore as stated, the distances y'BF and y'BN of the distance or near reference point from the apex of the lens have the following values:    Y'BF=4.0 mm, and y'BN=−14 mm
In other words, the main point for distance vision is 4 mm above the apex of the lens and the main point for near vision is 14 mm below the apex of the lens. Also stated in the specification, the refractive index function is a) solely a function of the coordinate y' so that by varying the refractive index, the increase in refractive power is produced only along the main meridian, or b) a function of the coordinate y' and x' so that not only the increase in refractive power along the main meridian, but also the correction of imaging errors on the main meridian and borne by the varying refractive index. Supported by the patent's drawings 4a and 4b respectively the index of refraction is shown in 4a to decrease below the main point for distance vision 4 mm above the apex of the lens to the main point for near vision at the −14 mm location, and well beyond. In fact the refractive index changes most dramatically below the −14 mm mark to the −20 mm mark (1.57 to 1.51[0.06 index units] over 6 mm) and comparatively least above the −14 mm mark to the 4 mm point for distance vision (1.57 to 1.604 [0.034 index units] over 18 mm). What this means is that the so-called reading portion has the most increase in refractive power change, and therefore fits more the definition of the intermediate portion, and the intermediate portion, from 4 mm to −14 mm, has comparatively the least increase in refractive power change, and therefore fits more the definition of the reading portion. A lens with altogether different refractive properties is needed to provide good optical qualities for a progressive ophthalmic lens.
U.S. Pat. No. 5,148,205 to Guilino et al. describes an ophthalmic lens having a front and an eye-facing boundary surface and a varying refractive index, which contributes to the correction of aberrations. The ophthalmic lens is distinguished by having at least one system of surfaces a given level (n(x,y,z)=const.) with a constant refractive index, which are spaced the same distance at all points in the direction of their surface normals (parallel surfaces), and which, respectively their extension, intersect the axis connecting the lens apexes of the front surface and the eye-facing surface. This patent describes a lens with a refractive index variation which depends on both the coordinate z lying in the direction of connecting axis of the apex of the lens and the coordinates x,y being perpendicular to the connecting axis, and therefore permits correcting aberrations and minimizing the critical lens thickness in a very simple manner. According to the specification, the gradients may be utilized for generating an astigmatic and/or progressive refractive power, with the design of the surface not or only partially contributing to the astigmatic and/or progressive refractive power. The bulk of the patent is directed to the use of what may be termed axial or modified axial refractive index gradients for the correction of aberrations and minimizing critical lens thickness. Only incidentally is there mention of the use of such refractive index gradients for a progressive addition spectacle lens. Such a design would appear to be very similar to that described in U.S. Pat. No. 5,042,936 to the same inventor filed less than one year earlier. Regardless whether the refractive index increases or decreases with increasing values of Z, such a progressive lens would suffer from similar or identical problems as the U.S. Pat. No. 5,042,936 lens referenced and described above.
U.S. Pat. No. 5,861,934 to Blum et al. describes a refractive index gradient lens comprising a composite of at least three different and separately applied layers, each layer having a different refractive index which allow for a progressive multifocal lens having a wide and natural progression of vision when looking from far to near. A transition zone disposed between a base and an outer layer includes a distinct and separately applied transition layer or layers having an effective refractive index which is intermediate between the refractive indices of the base and outer layers, and preferably approximates the geometric mean of the refractive indices of the base and outer layers. This transition zone may include multiple transition layers, with each transition layer having a different and distinct refractive index. In this lens invention the refractive indices of the base, outer and transition layer(s) are each constant throughout their respective layers. Included within the lens design is a region of varying thickness which defines a progressive multifocal zone. The technique of employing a transition zone having an intermediate refractive index is used in order to render the progressive multifocal area as invisible as possible. As stated in the patent, by way of example only, if the preform has a refractive index of about 1.50 and the outer layer has a refractive index of about 1.70, the refractive indices of three transition layers in a transition zone may be about 1.54, 1.60 and 1.66 as the layers progress from the preform to the outer layer. In this invention the gradient index does not contribute to the progressive power as in the previously mentioned prior art patents; rather, within the lens is a region of varying thickness which defines a progressive multifocal zone.
U.S. Pat. No. 6,942,339 to Dreher describes a multifocal or progressive lens constructed with a layer of variable index material, such as epoxy, sandwiched in between two lens blanks. The inner epoxy coating aberrator has vision zones configured to correct aberrations of the patient's eye and higher order aberrations. The variable index coating that comprises the inner layer of this lens does not provide the progressive add power of the lens, rather as stated in the patent it corrects for aberrations of the patient's eye. The lens has many of the limitations typical of aspheric progressive lenses.